Organic element for electroluminescent devices

ABSTRACT

Disclosed is a useful electroluminescent device comprising a cathode, an anode, and therebetween a light emitting layer containing a host material and a phosphorescent light-emitting material wherein the host material is represented by formula (1): 
 
X′-A-X″   (1) 
wherein: A is selected from the group consisting of an unsubstituted phenylene ring, a biphenylene group, a terphenylene group, a naphthylene group, and a fluorene group; and each of X′ and X″ is an independently selected aromatic group bearing an ortho aromatic substituent.

FIELD OF THE INVENTION

This invention relates to an organic light emitting diode (OLED)electroluminescent (EL) device comprising a light-emitting layercontaining an organometallic complex that can provide desirableelectroluminescent properties.

BACKGROUND OF THE INVENTION

While organic electroluminescent (EL) devices have been known for overtwo decades, their performance limitations have represented a barrier tomany desirable applications. In simplest form, an organic EL device iscomprised of an anode for hole injection, a cathode for electroninjection, and an organic medium sandwiched between these electrodes tosupport charge recombination that yields emission of light. Thesedevices are also commonly referred to as organic light-emitting diodes,or OLEDs. Representative of earlier organic EL devices are Gurnee et al.U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No.3,173,050, issued Mar. 9, 1965; Dresner, “Double InjectionElectroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334,1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Theorganic layers in these devices, usually composed of a polycyclicaromatic hydrocarbon, were very thick (much greater than 1 μm).Consequently, operating voltages were very high, often >100V.

More recent organic EL devices include an organic EL element consistingof extremely thin layers (e.g. <1.0 μm ) between the anode and thecathode. Herein, the term “organic EL element” encompasses the layersbetween the anode and cathode electrodes. Reducing the thickness loweredthe resistance of the organic layer and has enabled devices that operatemuch lower voltage. In a basic two-layer EL device structure, describedfirst in U.S. Pat. No. 4,356,429, one organic layer of the EL elementadjacent to the anode is specifically chosen to transport holes,therefore, it is referred to as the hole-transporting layer, and theother organic layer is specifically chosen to transport electrons,referred to as the electron-transporting layer. Recombination of theinjected holes and electrons within the organic EL element results inefficient electroluminescence.

There have also been proposed three-layer organic EL devices thatcontain an organic light-emitting layer (LEL) between thehole-transporting layer and electron-transporting layer, such as thatdisclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616,1989]. The light-emitting layer commonly consists of a host materialdoped with a guest material Still further, there has been proposed inU.S. Pat. No. 4,769,292 a four-layer EL element comprising ahole-injecting layer (HIL), a hole-transporting layer (HTL), alight-emitting layer (LEL) and an electron transport/injection layer(ETL). These structures have resulted in improved device efficiency.

Many emitting materials that have been described as useful in an OLEDdevice emit light from their excited singlet state by fluorescence. Theexcited singlet state is created when excitons formed in an OLED devicetransfer their energy to the excited state of the dopant. However, it isgenerally believed that only 25% of the excitons created in an EL deviceare singlet excitons. The remaining excitons are triplet, which cannotreadily transfer their energy to the singlet excited state of a dopant.This results in a large loss in efficiency since 75% of the excitons arenot used in the light emission process.

Triplet excitons can transfer their energy to a dopant if it has atriplet excited state that is low enough in energy. If the triplet stateof the dopant is emissive it can produce light by phosphorescence,wherein phosphorescence is a luminescence involving a change of spinstate between the excited state and the ground state. In many casessinglet excitons can also transfer their energy to lowest singletexcited state of the same dopant. The singlet excited state can oftenrelax, by an intersystem crossing process, to the emissive tripletexcited state. Thus, it is possible, by the proper choice of host anddopant, to collect energy from both the singlet and triplet excitonscreated in an OLED device and to produce a very efficient phosphorescentemission.

One class of useful phosphorescent materials are transition metalcomplexes having a triplet excited state. For example,fac-tris(2-phenylpyridinato-N,C^(2′))iridium(III) (Ir(ppy)₃) stronglyemits green light from a triplet excited state owing to the largespin-orbit coupling of the heavy atom and to the lowest excited statewhich is a charge transfer state having a Laporte allowed (orbitalsymmetry) transition to the ground state (K. A. King, P. J. Spellane,and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T.C. Brunold, T. Reidener, H. U. Gudel, M. Fortsch, and H.-B. Burgi,Inorg. Chem., 33, 545 (1994) Small-molecule, vacuum-deposited OLEDshaving high efficiency have also been demonstrated with Ir(ppy)₃ as thephosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as thehost (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M.Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S.Miyaguchi, Jpn. J Appl. Phys., 38, L1502 (1999)).

Another class of phosphorescent materials include compounds havinginteractions between atoms having d¹⁰ electron configuration, such asAu₂(dppm)Cl₂ (dppm=bis(diphenylphosphino)methane) (Y. Ma et al, Appl.Phys. Lett., 74, 1361 (1998)). Still other examples of usefulphosphorescent materials include coordination complexes of the trivalentlanthanides such as Tb³⁺ and Eu³⁺ (J. Kido et al, Appl. Phys. Lett., 65,2124 (1994)). While these latter phosphorescent compounds do notnecessarily have triplets as the lowest excited states, their opticaltransitions do involve a change in spin state of 1 and thereby canharvest the triplet excitons in OLED devices.

Suitable hosts for phosphorescent materials should be selected so thatthe triplet exciton can be transferred efficiently from the hostmaterial to the phosphorescent material. For example, host materials aredescribed in WO 00/70655 A2; 01/39234 A2; 01/93642 Al; 02/074015 A2;02/15645 A1, and U.S. Ser. No. 20020117662. T. Igarashi and X. Qiu alsodescribe host molecules in WO 2003/7658 and in particular host materialsfor phosphorescent dopants in U.S. Ser. No. 2003/39858.

Notwithstanding these developments, there remains a need for new hostmaterials, and especially hosts that will function with phosphorescentmaterials to provide improved efficiency, stability, manufacturability,or spectral characteristics of electroluminescent devices.

SUMMARY OF THE INVENTION

The invention provides an electroluminescent device comprising acathode, an anode, and therebetween a light emitting layer containing ahost material and a phosphorescent light-emitting material wherein thehost material is represented by formula (1):X′-A-X″   (1)wherein:

-   -   A is selected from the group consisting of an unsubstituted        phenylene ring, a biphenylene group, a terphenylene group, a        naphthylene group, and a fluorene group; and        -   each of X′ and X″ is an independently selected aromatic            group bearing an ortho aromatic substituent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a typical OLED device in which thisinvention may be used.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides an electroluminescent device comprising acathode, an anode, and therebetween a light emitting layer containing ahost material and a phosphorescent light-emitting material wherein thehost material is represented by formula (1):X′-A-X″   (1).

It is desirable to choose the structure of A, X′, and X″ so that thetriplet energy of the host material (formula 1) is higher than thetriplet energy of the phosphorescent light-emitting material. Suitably,A is selected from the group consisting of an unsubstituted phenylenering, wherein X′ and X″ may be attached para, ortho, or meta to oneanother. A may also represent a biphenylene group, a terphenylene group,a naphthylene group, or a fluorene group provided X′ and X″ are not onthe same ring. In one desirable embodiment, A represents a biphenylenegroup, a terphenylene group, or a fluorene group. In another desirableembodiment A represents the divalent form of one of the groups listedbelow.

In the formulas above each Z^(a) is an independently selectedsubstituent, for example a methyl group, or t-butyl group. When one ormore of Z^(a) represents an aromatic group, such as a phenyl group, itis desirable that the group have an substituent ortho to its point ofattachment so that it will be twisted in the excited triplet state. Eachn is independently 0 to 4, and each m is independently 0 to 3. In onesuitable embodiment, n and m are both 0. R^(a) and R^(b) independentlyrepresent substituents, for example methyl or ethyl groups.

Examples of A are listed below.

Each of X′ and X″, of formula 1, is an independently selected aromaticgroup bearing an ortho aromatic substituent. The ortho substituentinduces steric interactions between X′ and A as well as X″ and A, whichcauses twisting of X′ and X″ relative to A in the excited triplet state.This twisting raises the triplet energy of the host and provides moreefficient energy transfer from the host to the guest phosphorescentemitting material. Examples of X′ and X″ are listed below.

In one desirable embodiment each of X′ and X″ represent independentlyselected aromatic group each bearing two ortho aromatic substituents. Inone suitable embodiment the host material is represented by formula 1a.

In formula 1a, Ar¹-Ar⁴ independently represent aromatic groups, such asphenyl groups or tolyl groups. X¹-X⁶ independently represent hydrogen ora substituent. Examples of substituents are methyl groups, t-butylgroups, halogen, such as F, and aromatic groups such as phenyl groups.When one or more of X¹-X⁶ represents an aromatic group, it is desirablethat the group have a substituent ortho to its point of attachment sothat it will be twisted in the excited triplet state.

In another suitable embodiment, for example when the phosphorescentdopant is a green-light emitting material, each X′ and X″ do not havesubstituents that have more than one additional fused ring. For example,X′ and X″ are not substituted with anthracene groups or pyrene groups,since this can lower the triplet energy of the material and make it aninefficient host. In one desirable embodiment, for example when thephosphorescent dopant is a blue-light emitting material, X′ and X″ donot have substituents that have any filsed aromatic rings, for example,X′ and X″ are not substituted with naphthylene groups.

In one desirable embodiment the material of formula 1 is represented byformula 1b.

In formula 1b, Ar¹-Ar¹⁰ independently represent aromatic groups such asphenyl groups and tolyl groups.

Illustrative examples of X′ and X″ and materials of formula 1a and 1bare given below.

The invention provides an electroluminescent device comprising aphosphorescent light-emitting material. The phosphorescent material mayemit any color light, for example, blue, blue-green, green, orange orred light. In one suitable embodiment the phosphorescent materialcomprises an organometallic compound consisting of a metal selected fromIr, Rh, Os, Pt, and Pd, and an organic ligand. In one desirableembodiment the metal is Ir and the ligand is a phenylpyridine group.Illustrative examples of phosphorescent materials are given below.

In order to achieve efficient energy transfer from the host material tothe phosphorescent emitting material (guest material), it is desirablethat the triplet energy of the host material be equal or higher thanthat of the triplet energy of guest material. Phosphorescent materialsemitting light in the blue region will have a higher triplet energy thanthose emitting in the green region and, in turn, materials emittinglight in the green region will have a higher triplet energy than thoseemitting red light. The structure of the host material must be chosen sothat its triplet energy is sufficiently high that it can act as anefficient host. For materials of formula 1 that are to act as hosts forphosphorescent materials that emit blue light it is desirable that X′,A, and X″ do not have any groups present that have fused rings. In thecase where materials of formula 1 are to act as hosts for greenphosphorescent materials, it is desirable that X′, A, and X″ are chosenso that there are no groups present that have more than one fusedaromatic ring.

The triplet energy for a molecule is defined as the difference betweenthe ground state energy (E(gs)) of the molecule and the energy of thelowest triplet state (E(ts)) of the molecule, both given in eV. Theseenergies can be calculated using the B3LYP method as implemented in theGaussian98 (Gaussian, Inc., Pittsburgh, Pa.) computer program. The basisset for use with the B3LYP method is defined as follows: MIDI! for allatoms for which MIDI! is defined, 6-31 G* for all atoms defined in6-31G* but not in MIDI!, and either the LACV3P or the LANL2DZ basis setand pseudopotential for atoms not defined in MIDI! or 6-31 G*, withLACV3P being the preferred method. For any remaining atoms, anypublished basis set and pseudopotential may be used. MIDI!, 6-31 G* andLANL2DZ are used as implemented in the Gaussian98 computer code andLACV3P is used as implemented in the Jaguar 4.1 (Schrodinger, Inc.,Portland Oreg.) computer code. The energy of each state is computed atthe minimum-energy geometry for that state. The difference in energybetween the two states is further modified by Equation 1 to give thetriplet energy (E(t)):E(t)=0.84*(E(ts)−E(gs))+0.35   (1)

For polymeric or oligomeric materials, it is sufficient to compute thetriplet energy over a monomer or oligomer of sufficient size so thatadditional units do not substantially change the computed triplet energyof the phosphorescent light emitting material.

In one suitable embodiment the light emitting layer of the invention hasan adjacent layer that is an exciton or hole blocking layer to helpconfine the excitons or electron-hole recombination centers to thelight-emitting layer comprising the host and phosphorescent material. Inone embodiment this layer comprises an aluminum complex of2-methyl-8-hydroxyquinoline.

The host material(s) is usually present in an amount more than theamount of dopant materials. Typically, the dopant material is present inan amount of up to 15.0 wt % of the host, more typically from 0.1-10.0wt % of the host, and commonly 2-8 wt % of the host. The host materialof the invention is present in 10-99.9 wt % of the phosphorescentemissive layer, more typically in 25-99 wt %, and more commonly 85-99 wt% of the phosphorescent emissive layer. In one suitable embodiment, thehost material of the invention is used in combination with a second hostmaterial. In one desirable embodiment the second host material comprisesa carbazole ring, for example the second host can be4,4′-bis(carbazol-9-yl)biphenyl. Suitably, the second host material ispresent at 25-75 wt % of the phosphorescent emissive layer.

The materials of formula 1 useful in the invention may be synthesized byvarious literature methods. For example, see R. Pascal, N. Hayashi, D.Ho, Tetrahedron, 57, 3549 (2001), L. Tong, H. Lau, H. Heidi, M. Douglas,R. Pascal, J. Amer. Chem. Soc., 120, 6000 (1998), Y. Fujioka, S. Ozasa,K. Sato, E. Ibuki, Chem. & Pharm. Bull. 33, 22 (1985), S. Ito, M.Wehmeier, J. Brand, C. Kubel, R. Epsch, J. Rabe, K. Müllen, Chem.-Eur.J., 6,4327 (2000), M. Muller, C. Kubel, F. Morgenroth, V. Iyer, K.Müllen, Carbon, 36, 827 (1998) and references cited therein.

Embodiments of the invention can provide advantageous features such asoperating efficiency, higher luminance, color hue, low drive voltage,and improved operating stability. Embodiments of the organometalliccompounds useful in the invention can provide a wide range of huesincluding those useful in the emission of white light (directly orthrough filters to provide multicolor displays).

Illustrative examples of complexes of the materials of formula (1)useful in the present invention are the following:

Unless otherwise specifically stated, use of the term “substituted” or“substituent” means any group or atom other than hydrogen. Unlessotherwise 5 provided, when a group (including a compound or complex)containing a substitutable hydrogen is referred to, it is also intendedto encompass not only the unsubstituted form, but also form furthersubstituted derivatives with any substituent group or groups as hereinmentioned, so long as the substituent does not destroy propertiesnecessary for utility. Suitably, a substituent group may be lo halogenor may be bonded to the remainder of the molecule by an atom of carbon,silicon, oxygen, nitrogen, phosphorous, sulfur, selenium, or boron. Thesubstituent may be, for example, halogen, such as chloro, bromo orfluoro; nitro; hydroxyl; cyano; carboxyl; or groups which may be furthersubstituted, such as alkyl, including straight or branched chain orcyclic alkyl, such as methyl, trifluoromethyl, ethyl, t-butyl,3-(2,4-di-t-pentylphenoxy) propyl, and tetradecyl; alkenyl, such asethylene, 2-butene; alkoxy, such as methoxy, ethoxy, propoxy, butoxy,2-methoxyethoxy, sec-butoxy, hexyloxy, 2-ethylhexyloxy, tetradecyloxy,2-(2,4-di-t-pentylphenoxy)ethoxy, and 2-dodecyloxyethoxy; aryl such asphenyl, 4-t-butylphenyl, 2,4,6-trimethylphenyl, naphthyl; aryloxy, suchas phenoxy, 2-methylphenoxy, alpha- or beta-naphthyloxy, and 4-tolyloxy;carbonamido, such as acetamido, benzamido, butyramido, tetradecanamido,alpha-(2,4-di-t-pentyl-phenoxy)acetamido,alpha-(2,4-di-t-pentylphenoxy)butyramido,alpha-(3-pentadecylphenoxy)-hexanamido,alpha-(4-hydroxy-3-t-butylphenoxy)-tetradecanamido,2-oxo-pyrrolidin-1-yl, 2-oxo-5-tetradecylpyrrolin-1-yl,N-methyltetradecanamido, N-succinimido, N-phthalimido,2,5-dioxo-1-oxazolidinyl, 3-dodecyl-2,5-dioxo-1-imidazolyl, andN-acetyl-N-dodecylamino, ethoxycarbonylamino, phenoxycarbonylamino,benzyloxycarbonylamino, hexadecyloxycarbonylamino,2,4-di-t-butylphenoxycarbonylamino, phenylcarbonylamino,2,5-(di-t-pentylphenyl)carbonylamino, p-dodecyl-phenylcarbonylamino,p-tolylcarbonylamino, N-methylureido, N,N-dimethylureido,N-methyl-N-dodecylureido, N-hexadecylureido, N,N-dioctadecylureido,N,N-dioctyl-N′-ethylureido, N-phenylureido, N,N-diphenylureido,N-phenyl-N-p-tolylureido, N-(m-hexadecylphenyl)ureido,N,N-(2,5-di-t-pentylphenyl)-N′-ethylureido, and t-butylcarbonamido;sulfonamido, such as methylsulfonamido, benzenesulfonamido,p-tolylsulfonamido, p-dodecylbenzenesulfonamido,N-methyltetradecylsulfonamido, N,N-dipropyl-sulfamoylamino, andhexadecylsulfonamido; sulfamoyl, such as N-methylsulfamoyl,N-ethylsulfamoyl, N,N-dipropylsulfamoyl, N-hexadecylsulfamoyl,N,N-dimethylsulfamoyl, N-[3-(dodecyloxy)propyl]sulfamoyl,N-[4-(2,4-di-t-pentylphenoxy)butyl]sulfamoyl,N-methyl-N-tetradecylsulfamoyl, and N-dodecylsulfamoyl; carbamoyl, suchas N-methylcarbamoyl, N,N-dibutylcarbamoyl, N-octadecylcarbamoyl,N-[4-(2,4-di-t-pentylphenoxy)butyl]carbamoyl,N-methyl-N-tetradecylcarbamoyl, and N,N-dioctylcarbamoyl; acyl, such asacetyl, (2,4-di-t-amylphenoxy)acetyl, phenoxycarbonyl,p-dodecyloxyphenoxycarbonyl methoxycarbonyl, butoxycarbonyl,tetradecyloxycarbonyl, ethoxycarbonyl, benzyloxycarbonyl,3-pentadecyloxycarbonyl, and dodecyloxycarbonyl; sulfonyl, such asmethoxysulfonyl, octyloxysulfonyl, tetradecyloxysulfonyl,2-ethylhexyloxysulfonyl, phenoxysulfonyl,2,4-di-t-pentylphenoxysulfonyl, methylsulfonyl, octylsulfonyl,2-ethylhexylsulfonyl, dodecylsulfonyl, hexadecylsulfonyl,phenylsulfonyl, 4-nonylphenylsulfonyl, and p-tolylsulfonyl; sulfonyloxy,such as dodecylsulfonyloxy, and hexadecylsulfonyloxy; sulfinyl, such asmethylsulfinyl, octylsulfinyl, 2-ethylhexylsulfinyl, dodecylsulfinyl,hexadecylsulfinyl, phenylsulfinyl, 4-nonylphenylsulfinyl, andp-tolylsulfinyl; thio, such as ethylthio, octylthio, benzylthio,tetradecylthio, 2-(2,4-di-t-pentylphenoxy)ethylthio, phenylthio,2-butoxy-5-t-octylphenylthio, and p-tolylthio; acyloxy, such asacetyloxy, benzoyloxy, octadecanoyloxy, p-dodecylamidobenzoyloxy,N-phenylcarbamoyloxy, N-ethylcarbamoyloxy, and cyclohexylcarbonyloxy;amine, such as phenylanilino, 2-chloroanilino, diethylamine,dodecylamine; imino, such as 1 (N-phenylimido)ethyl, N-succinimido or3-benzylhydantoinyl; phosphate, such as dimethylphosphate andethylbutylphosphate; phosphite, such as diethyl and dihexylphosphite; aheterocyclic group, a heterocyclic oxy group or a heterocyclic thiogroup, each of which may be substituted and which contain a 3 to 7membered heterocyclic ring composed of carbon atoms and at least onehetero atom selected from the group consisting of oxygen, nitrogen,sulfur, phosphorous, or boron. such as 2-fliryl, 2-thienyl,2-benzimidazolyloxy or 2-benzothiazolyl; quaternary ammonium, such astriethylammonium; quaternary phosphonium, such as triphenylphosphonium;and silyloxy, such as trimethylsilyloxy.

If desired, the substituents may themselves be further substituted oneor more times with the described substituent groups. The particularsubstituents used may be selected by those skilled in the art to attainthe desired desirable properties for a specific application and caninclude, for example, electron-withdrawing groups, electron-donatinggroups, and steric groups. When a molecule may have two or moresubstituents, the substituents may be joined together to form a ringsuch as a fused ring unless otherwise provided. Generally, the abovegroups and substituents thereof may include those having up to 48 carbonatoms, typically 1 to 36 carbon atoms and usually less than 24 carbonatoms, but greater numbers are possible depending on the particularsubstituents selected.

Suitably, the light-emitting layer of the OLED device comprises a hostmaterial and one or more guest materials for emitting light. At leastone of the host materials is suitably a compound comprising a ringsystem of formula 1. The light-emitting guest material(s) is usuallypresent in an amount less than the amount of host materials and istypically present in an amount of up to 15 wt % of the host, moretypically from 0.1-10 wt % of the host, and commonly from 2-8% of thehost. For convenience, the phosphorescent complex guest material may bereferred to herein as a phosphorescent material. The host material offormula 1 is preferably a low molecular weight compound, but it may alsobe an oligomer or a polymer having a main chain or a side chain ofrepeating units having the moiety represented by formula 1.

Other Host Materials for Phosphorescent Materials

The host material useful in the invention may be used alone or incombination with other host materials. Other host materials should beselected so that the triplet exciton can be transferred efficiently fromthe host material to the phosphorescent material. For this transfer tooccur, it is a highly desirable condition that the excited state energyof the phosphorescent material be lower than the difference in energybetween the lowest triplet state and the ground state of the host.However, the band gap of the host should not be chosen so large as tocause an unacceptable increase in the drive voltage of the OLED.Suitable host materials are described in WO 00/70655 A2; 01/39234 A2;01/93642 A1; 02/074015 A2; 02/15645 A1, and U.S. Ser. No. 20020117662.Suitable hosts include certain aryl amines, triazoles, indoles andcarbazole compounds. Examples of desirable hosts are4,4′-N,N′-dicarbazole-biphenyl (CBP),2,2′-dimethyl-4,4′-N,N′-dicarbazole-biphenyl,m-(N,N′-dicarbazole)benzene, and poly(N-vinylcarbazole), including theirderivatives.

Desirable host materials are capable of forming a continuous film. Thelight-emitting layer may contain more than one host material in order toimprove the device's film morphology, electrical properties, lightemission efficiency, and lifetime. The light emitting layer may containa first host material that has good hole-transporting properties, and asecond host material that has good electron-transporting properties.

Phosphorescent Materials

Phosphorescent materials may be used alone or, in certain cases, incombination with each other, either in the same or different layers.Examples of phosphorescent and related materials are described in WO00/57676, WO 00/70655, WO 01/41512 A1, WO 02/15645 A1, U.S. Ser. No.2003/0017361 A1, WO 01/93642 A1, WO 01/39234 A2, U.S. Pat. No. 6,458,475Bl, WO 02/071813 A1, U.S. Pat. No. 6,573,651 B2, U.S. Ser. No.2002/0197511 A1, WO 02/074015 A2, U.S. Pat. No. 6,451,455 B1, U.S. Ser.No. 2003/0072964 A1, U.S. Ser. No. 2003/0068528 A1, U.S. Pat. No.6,413,656 B1, U.S. Pat. No. 6,515,298 B2, U.S. Pat. No. 6,451,415 B1,U.S. Pat. No. 6,097,147, U.S. Ser. No. 2003/0124381 A1, U.S. Ser. No.2003/0059646 A1, U.S. Ser. No. 2003/0054198 A1, EP 1 239 526 A2, EP 1238 981 A2, EP 1 244 155 A2, U.S. Ser. No. 2002/0100906 A1, U.S. Ser.No. 2003/0068526 A1, U.S. Pat. No. 2003/0068535 A1, JP 2003073387A, JP2003 073388A, U.S. Ser. No. 2003/0141809 A1, U.S. Ser. No. 2003/0040627A1, JP 2003059667A, JP 2003073665A, and U.S. Ser. No. 2002/0121638 A1.

The emission wavelengths of cyclometallated Ir(IHl) complexes of thetype IrL₃ and IrL₂L′, such as the green-emittingfac-tris(2-phenylpyridinato-N,C^(2′))Iridium(III) andbis(2-phenylpyridinato-N,C^(2′))Iridium(III)(acetylacetonate) may beshifted by substitution of electron donating or withdrawing groups atappropriate positions on the cyclometallating ligand L, or by choice ofdifferent heterocycles for the cyclometallating ligand L. The emissionwavelengths may also be shifted by choice of the ancillary ligand L′.Examples of red emitters are thebis(2-(2′-benzothienyl)pyridinato-N,C^(3′))Iridium(El)(acetylacetonate)and tris(1-phenylisoquinolinato-N,C)Iridium(III). A blue-emittingexample isbis(2-(4,6-diflourophenyl)-pyridinato-N,C^(2′))Iridium(III)(picolinate).

Red electrophosphorescence has been reported, usingbis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C³)iridium(acetylacetonate)[Btp₂Ir(acac)] as the phosphorescent material(Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E.,and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001).

Other important phosphorescent materials include cyclometallated Pt(II)complexes such as cis-bis(2-phenylpyridinato-N,C^(2′))platinum(II),cis-bis(2-(2′-thienyl)pyridinato-N,C^(3′)) platinum(II),cis-bis(2-(2′-thienyl)quinolinato-N,C^(5′)) platinum(II), or(2-(4,6-diflourophenyl)pyridinato-NC2′) platinum(II)acetylacetonate.Pt(II)porphyrin complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(H) are also useful phosphorescent materials.

Still other examples of useful phosphorescent materials includecoordination complexes of the trivalent lanthanides such as Th³⁺ andEu³⁺ (J. Kido et al, Appl. Phys. Lett., 65, 2124 (1994))

Blocking Layers

In addition to suitable hosts, an OLED device employing a phosphorescentmaterial often requires at least one exciton or hole blocking layers tohelp confine the excitons or electron-hole recombination centers to thelight-emitting layer comprising the host and phosphorescent material. Inone embodiment, such a blocking layer would be placed between theelectron-transporting layer and the light-emitting layer—see FIG. 1,layer 110. In this case, the ionization potential of the blocking layershould be such that there is an energy barrier for hole migration fromthe host into the electron-transporting layer, while the electronaffinity should be such that electrons pass more readily from theelectron-transporting layer into the light-emitting layer comprisinghost and phosphorescent material. It is further desired, but notabsolutely required, that the triplet energy of the blocking material begreater than that of the phosphorescent material. Suitable hole-blockingmaterials are described in WO 00/70655A2 and WO 01/93642 A1. Twoexamples of useful materials are bathocuproine (BCP) andbis(2-methyl-8-quinolinolato)(4-phenylphenolato)Aluminum(III) (BA1Q).Metal complexes other than Ba1q are also known to block holes andexcitons as described in U.S. Ser. No. 20030068528. U.S. Ser. No.20030175553 A1 describes the use of fac-tris(1-phenylpyrazolato-N,C2)iridium(III) (Irppz) in an electron/exciton blocking layer.

Embodiments of the invention can provide advantageous features such asoperating efficiency, higher luminance, color hue, low drive voltage,and improved operating stability. Embodiments of the organometalliccompounds useful in the invention can provide a wide range of huesincluding those useful in the emission of white light (directly orthrough filters to provide multicolor displays).

General Device Architecture

The present invention can be employed in many OLED device configurationsusing small molecule materials, oligomeric materials, polymericmaterials, or combinations thereof. These include very simple structurescomprising a single anode and cathode to more complex devices, such aspassive matrix displays comprised of orthogonal arrays of anodes andcathodes to form pixels, and active-matrix displays where each pixel iscontrolled independently, for example, with thin film transistors(TFTs).

There are numerous configurations of the organic layers wherein thepresent invention can be successfully practiced. The essentialrequirements of an OLED are an anode, a cathode, and an organiclight-emitting layer located between the anode and cathode. Additionallayers may be employed as more fully described hereafter.

A typical structure, especially useful for of a small molecule device,is shown in FIG. 1 and is comprised of a substrate 101, an anode 103, ahole-injecting layer 105, a hole-transporting layer 107, alight-emitting layer 109, a hole- or exciton-blocking layer 110, anelectron-transporting layer 111, and a cathode 113. These layers aredescribed in detail below. Note that the substrate may alternatively belocated adjacent to the cathode, or the substrate may actuallyconstitute the anode or cathode. The organic layers between the anodeand cathode are conveniently referred to as the organic EL element.Also, the total combined thickness of the organic layers is desirablyless than 500 nm.

The anode and cathode of the OLED are connected to a voltage/currentsource through electrical conductors. The OLED is operated by applying apotential between the anode and cathode such that the anode is at a morepositive potential than the cathode. Holes are injected into the organicEL element from the anode and electrons are injected into the organic ELelement at the cathode. Enhanced device stability can sometimes beachieved when the OLED is operated in an AC mode where, for some timeperiod in the cycle, the potential bias is reversed and no currentflows. An example of an AC driven OLED is described in U.S. Pat. No.5,552,678.

Substrate The OLED device of this invention is typically provided over asupporting substrate 101 where either the cathode or anode can be incontact with the substrate. The electrode in contact with the substrateis conveniently referred to as the bottom electrode. Conventionally, thebottom electrode is the anode, but this invention is not limited to thatconfiguration. The substrate can either be light transmissive or opaque,depending on the intended direction of light emission. The lighttransmissive property is desirable for viewing the EL emission throughthe substrate. Transparent glass or plastic is commonly employed in suchcases. The substrate can be a complex structure comprising multiplelayers of materials. This is typically the case for active matrixsubstrates wherein TFTs are provided below the OLED layers. It is stillnecessary that the substrate, at least in the emissive pixilated areas,be comprised of largely transparent materials such as glass or polymers.For applications where the EL emission is viewed through the topelectrode, the transmissive characteristic of the bottom support isimmaterial, and therefore can be light transmissive, light absorbing orlight reflective. Substrates for use in this case include, but are notlimited to, glass, plastic, semiconductor materials, silicon, ceramics,and circuit board materials. Again, the substrate can be a complexstructure comprising multiple layers of materials such as found inactive matrix TFT designs. It is necessary to provide in these deviceconfigurations a light-transparent top electrode.

Anode

When the desired electroluminescent light emission (EL) is viewedthrough the anode, the anode should be transparent or substantiallytransparent to the emission of interest. Common transparent anodematerials used in this invention are indium-tin oxide (ITO), indium-zincoxide (IZO) and tin oxide, but other metal oxides can work including,but not limited to, aluminum- or indium-doped zinc oxide,magnesium-indium oxide, and nickel-tungsten oxide. In addition to theseoxides, metal nitrides, such as gallium nitride, and metal selenides,such as zinc selenide, and metal sulfides, such as zinc sulfide, can beused as the anode. For applications where EL emission is viewed onlythrough the cathode, the transmissive characteristics of the anode areimmaterial and any conductive material can be used, transparent, opaqueor reflective. Example conductors for this application include, but arenot limited to, gold, iridium, molybdenum, palladium, and platinum.Typical anode materials, transmissive or otherwise, have a work functionof 4.1 eV or greater. Desired anode materials are commonly deposited byany suitable means such as evaporation, sputtering, chemical vapordeposition, or electrochemical means. Anodes can be patterned usingwell-known photolithographic processes. Optionally, anodes may bepolished prior to application of other layers to reduce surfaceroughness so as to minimize shorts or enhance reflectivity.

Cathode

When light emission is viewed solely through the anode, the cathode usedin this invention can be comprised of nearly any conductive material.Desirable materials have good film-forming properties to ensure goodcontact with the underlying organic layer, promote electron injection atlow voltage, and have good stability. Useful cathode materials oftencontain a low work function metal (<4.0 eV) or metal alloy. One usefulcathode material is comprised of a Mg:Ag alloy wherein the percentage ofsilver is in the range of 1 to 20%, as described in U.S. Pat. No.4,885,221. Another suitable class of cathode materials includes bilayerscomprising the cathode and a thin electron-injection layer (EIL) incontact with an organic layer (e.g., an electron transporting layer(ETL)) which is capped with a thicker layer of a conductive metal. Here,the EIL preferably includes a low work function metal or metal salt, andif so, the thicker capping layer does not need to have a low workfunction. One such cathode is comprised of a thin layer of LiF followedby a thicker layer of Al as described in U.S. Pat. No. 5,677,572. An ETLmaterial doped with an alkali metal, for example, Li-doped A1q, isanother example of a useful EIL. Other useful cathode material setsinclude, but are not limited to, those disclosed in U.S. Pat. Nos.5,059,861, 5,059,862, and 6,140,763.

When light emission is viewed through the cathode, the cathode must betransparent or nearly transparent. For such applications, metals must bethin or one must use transparent conductive oxides, or a combination ofthese materials. Optically transparent cathodes have been described inmore detail in U.S. Pat. No. 4,885,211, U.S. Pat. No. 5,247,190, JP3,234,963, U.S. Pat. No. 5,703,436, U.S. Pat. No. 5,608,287, U.S. Pat.No. 5,837,391, U.S. Pat. No. 5,677,572, U.S. Pat. No. 5,776,622, U.S.Pat. No. 5,776,623, U.S. Pat. No. 5,714,838, U.S. Pat. No. 5,969,474,U.S. Pat. No. 5,739,545, U.S. Pat. No. 5,981,306, U.S. Pat. No.6,137,223, U.S. Pat. No. 6,140,763, U.S. Pat. No. 6,172,459, EP 1 076368, U.S. Pat. No. 6,278,236, and U.S. Pat. No. 6,284,3936. Cathodematerials are typically deposited by any suitable method such asevaporation, sputtering, or chemical vapor deposition. When needed,patterning can be achieved through many well known methods including,but not limited to, through-mask deposition, integral shadow masking asdescribed in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation,and selective chemical vapor deposition.

Hole-Injecting Layer (HIL)

A hole-injecting layer 105 may be provided between anode 103 andhole-transporting layer 107. The hole-injecting material can serve toimprove the film formation property of subsequent organic layers and tofacilitate injection of holes into the hole-transporting layer. Suitablematerials for use in the hole-injecting layer include, but are notlimited to, porphyrinic compounds as described in U.S. Pat. No.4,720,432, plasma-deposited fluorocarbon polymers as described in U.S.Pat. No. 6,208,075, and some aromatic amines, for example, m-MTDATA(4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternativehole-injecting materials reportedly useful in organic EL devices aredescribed in EP 0 891 121 Al and EP 1 029 909 A1.

Hole-Transporting Laver (HTL)

The hole-transporting layer 107 of the organic EL device contains atleast one hole-transporting compound such as an aromatic tertiary amine,where the latter is understood to be a compound containing at least onetrivalent nitrogen atom that is bonded only to carbon atoms, at leastone of which is a member of an aromatic ring. In one form the aromatictertiary amine can be an arylamine, such as a monoarylamine,diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomerictriarylamines are illustrated by Klupfel et al. U.S. Pat. No. 3,180,730.Other suitable triarylamines substituted with one or more vinyl radicalsand/or comprising at least one active hydrogen containing group aredisclosed by Brantley et al U.S. Pat. No. 3,567,450 and U.S. Pat. No.3,658,520.

A more preferred class of aromatic tertiary amines are those whichinclude at least two aromatic tertiary amine moieties as described inU.S. Pat. No. 4,720,432 and U.S. Pat. No. 5,061,569. Such compoundsinclude those represented by structural formula (A).

wherein Q₁ and Q₂ are independently selected aromatic tertiary aminemoieties and G is a linking group such as an arylene, cycloalkylene, oralkylene group of a carbon to carbon bond. In one embodiment, at leastone of Q₁ or Q₂ contains a polycyclic fused ring structure, e.g., anaphthalene. When G is an aryl group, it is conveniently a phenylene,biphenylene, or naphthalene moiety.

A useful class of triarylamines satisfying structural formula (A) andcontaining two triarylamine moieties is represented by structuralformula (B):

where

-   -   R₁ and R₂ each independently represents a hydrogen atom, an aryl        group, or an alkyl group or R₁ and R₂ together represent the        atoms completing a cycloalkyl group; and    -   R₃ and R₄ each independently represents an aryl group, which is        in turn substituted with a diaryl substituted amino group, as        indicated by structural formula (C):        wherein R₅ and R₆ are independently selected aryl groups. In one        embodiment, at least one of R₅ or R₆ contains a polycyclic fused        ring structure, e.g., a naphthalene.

Another class of aromatic tertiary amines are the tetraaryldiamines.Desirable tetraaryldiamines include two diarylamino groups, such asindicated by formula (C), linked through an arylene group. Usefultetraaryldiamines include those represented by formula (D).

wherein

-   -   each Are is an independently selected arylene group, such as a        phenylene or anthracene moiety,    -   n is an integer of from 1 to 4, and    -   Ar, R₇, R₈, and R₉ are independently selected aryl groups.

In a typical embodiment, at least one of Ar, R₇, R₈, and R₉ is apolycyclic fused ring structure, e.g., a naphthalene

The various alkyl, alkylene, aryl, and arylene moieties of the foregoingstructural formulae (A), (B), (C), (D), can each in turn be substituted.Typical substituents include alkyl groups, alkoxy groups, aryl groups,aryloxy groups, and halogen such as fluoride, chloride, and bromide. Thevarious alkyl and alkylene moieties typically contain from about 1 to 6carbon atoms. The cycloalkyl moieties can contain from 3 to about 10carbon atoms, but typically contain five, six, or seven ring carbonatoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures.The aryl and arylene moieties are usually phenyl and phenylene moieties.

The hole-transporting layer can be formed of a single or a mixture ofaromatic tertiary amine compounds. Specifically, one may employ atriarylamine, such as a triarylamine satisfying the formula (B), incombination with a tetraaryldiamine, such as indicated by formula (D).When a triarylamine is employed in combination with a tetraaryldiamine,the latter is positioned as a layer interposed between the triarylamineand the electron injecting and transporting layer. Illustrative ofuseful aromatic tertiary amines are the following:

-   1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane-   1,1-Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane-   N,N,N′,N′-tetraphenyl-4,4′″-diamino-1,1′:4′,1″:4″, 1′″-quaterphenyl-   Bis(4-dimethylamino-2-methylphenyl)phenylmethane-   1,4-bis[2-[4-[N,N-di(p-toly)amino]phenyl]vinyl]benzene(BDTAPVB)-   N,N,N′,N′-Tetra-p-tolyl-4,4′-diaminobiphenyl-   N,N,N′,N′-Tetraphenyl-4,4′-diaminobiphenyl-   N,N,N′,N′-tetra-1-naphthyl-4,4′-diaminobiphenyl-   N,N,N′,N′-tetra-2-naphthyl-4,4′-diaminobiphenyl-   N-Phenylcarbazole-   4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl(NPB)-   4,4′-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl(TNB)-   4,4′-Bis[N-(1-naphthyl)-N-phenylamino]p-terphenyl-   4,4′-Bis[N-(2-naphthyl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl-   1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene-   4,4′-Bis[N-(9-anthryl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(1-anthryl)-N-phenylamino]-p-terphenyl-   4,4′-Bis[N-(2-phenanthryl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(2-pyrenyl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(2-perylenyl)-N-phenylamino]biphenyl-   4,4′-Bis[N-(1-coronenyl)-N-phenylamino]biphenyl-   2,6-Bis(di-p-tolylamino)naphthalene-   2,6-Bis[di-(1-naphthyl)amino]naphthalene-   2,6-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene-   N,N,N′,N′-Tetra(2-naphthyl)-4,4″-diamino-p-terphenyl-   4,4′-Bis {N-phenyl-N-[4-(1-naphthyl)-phenyl]amino}biphenyl-   2,6-Bis[N,N-di(2-naphthyl)amino]fluorene-   4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (MTDATA)-   4,4′-Bis[N-(3-methylphenyl)-N-phenylamino]biphenyl(TPD)

Another class of useful hole-transporting materials includes polycyclicaromatic compounds as described in EP 1 009 041. Tertiary aromaticamines with more than two amine groups may be used including oligomericmaterials. In addition, polymeric hole-transporting materials can beused such as poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole,polyaniline, and copolymers such aspoly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also calledPEDOT/PSS.

Fluorescent Light-Emitting Materials and Layers (LEL)

In addition to the phosphorescent materials of this invention, otherlight emitting materials may be used in the OLED device, includingfluorescent materials. Although the term “fluorescent” is commonly usedto describe any light emitting material, in this case we are referringto a material that emits light from a singlet excited state. Fluorescentmaterials may be used in the same layer as the phosphorescent material,in adjacent layers, in adjacent pixels, or any combination. Care must betaken not to select materials that will adversely affect the performanceof the phosphorescent materials of this invention. One skilled in theart will understand that triplet excited state energies of materials inthe same layer as the phosphorescent material or in an adjacent layermust be appropriately set so as to prevent unwanted quenching.

As more fully described in U.S. Pat. Nos. 4,769,292 and 5,935,721, thelight-emitting layer (LEL) of the organic EL element includes aluminescent fluorescent or phosphorescent material whereelectroluminescence is produced as a result of electron-hole pairrecombination in this region. The light-emitting layer can be comprisedof a single material, but more commonly consists of a host materialdoped with a guest emitting material or materials where light emissioncomes primarily from the emitting materials and can be of any color. Thehost materials in the light-emitting layer can be anelectron-transporting material, as defined below, a hole-transportingmaterial, as defined above, or another material or combination ofmaterials that support hole-electron recombination. . Fluorescentemitting materials are typically incorporated at 0.01 to 10% by weightof the host material.

The host and emitting materials can be small non-polymeric molecules orpolymeric materials such as polyfluorenes and polyvinylarylenes (e.g.,poly(p-phenylenevinylene), PPV). In the case of polymers, small moleculeemitting materials can be molecularly dispersed into a polymeric host,or the emitting materials can be added by copolymerizing a minorconstituent into a host polymer. Host materials may be mixed together inorder to improve film formation, electrical properties, light emissionefficiency, lifetime, or manufacturability. The host may comprise amaterial that has good hole-transporting properties and a material thathas good electron-transporting properties.

An important relationship for choosing a fluorescent dye as a guestemitting material is a comparison of the singlet excited state energiesof the host and light-emitting material. For efficient energy transferfrom the host to the emitting material, a highly desirable condition isthat the singlet excited state energy of the emitting material is lowerthan that of the host material

Host and emitting materials known to be of use include, but are notlimited to, those disclosed in U.S. Pat. No. 4,768,292, U.S. Pat. No.5,141,671, U.S. Pat. No. 5,150,006, U.S. Pat. No. 5,151,629, U.S. Pat.No. 5,405,709, U.S. Pat. No. 5,484,922, U.S. Pat. No. 5,593,788, U.S.Pat. No. 5,645,948, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,755,999,U.S. Pat. No. 5,928,802, U.S. Pat. No. 5,935,720, U.S. Pat. No.5,935,721, and U.S. Pat. No. 6,020,078.

Metal complexes of 8-hydroxyquinoline and similar derivatives (FormulaE) constitute one class of useful host compounds capable of supportingelectroluminescence, and are particularly suitable for light emission ofwavelengths longer than 500 nm, e.g., green, yellow, orange, and red.

wherein

-   -   M represents a metal;    -   n is an integer of from 1 to 4; and    -   Z independently in each occurrence represents the atoms        completing a nucleus having at least two fused aromatic rings.

From the foregoing it is apparent that the metal can be monovalent,divalent, trivalent, or tetravalent metal. The metal can, for example,be an alkali metal, such as lithium, sodium, or potassium; an alkalineearth metal, such as magnesium or calcium; an earth metal, such aluminumor gallium, or a transition metal such as zinc or zirconium. Generallyany monovalent, divalent, trivalent, or tetravalent metal known to be auseful chelating metal can be employed.

Z completes a heterocyclic nucleus containing at least two fusedaromatic rings, at least one of which is an azole or azine ring.Additional rings, including both aliphatic and aromatic rings, can befused with the two required rings, if required. To avoid addingmolecular bulk without improving on fuiction the number of ring atoms isusually maintained at 18 or less.

Illustrative of useful chelated oxinoid compounds are the following:

-   -   CO-1: Aluminum trisoxine [alias,        tris(8-quinolinolato)aluminum(IIII)]    -   CO-2: Magnesium bisoxine [alias,        bis(8-quinolinolato)magnesium(II)]    -   CO-3: Bis[benzo{f}-8-quinolinolato]zinc(II)    -   CO-4:        Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)    -   CO-5: Indium trisoxine [alias, tris(8-quinolinolato)indium]    -   CO-6: Aluminum tris(5-methyloxine) [alias,        tris(5-methyl-8-quinolinolato) aluminum(III)]    -   CO-7: Lithium oxine [alias, (8-quinolinolato)lithium(I)]    -   CO-8: Gallium oxine [alias, tris(8-quinolinolato)gallium(III)]    -   CO-9: Zirconium oxine [alias,        tetra(8-quinolinolato)zirconium(IV)]

Derivatives of 9,10-di-(2-naphthyl)anthracene (Formula F) constitute oneclass of useful host materials capable of supportingelectroluminescence, and are particularly suitable for light emission ofwavelengths longer than 400 nm, e.g., blue, green, yellow, orange orred.

wherein: R¹, R², R³, R⁴, R⁵, and R⁶ represent one or more substituentson each ring where each substituent is individually selected from thefollowing groups:

-   -   Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;    -   Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms;    -   Group 3: carbon atoms from 4 to 24 necessary to complete a fused        aromatic ring of anthracenyl; pyrenyl, or perylenyl;    -   Group 4: heteroaryl or substituted heteroaryl of from 5 to 24        carbon atoms as necessary to complete a fused heteroaromatic        ring of furyl, thienyl, pyridyl, quinolinyl or other        heterocyclic systems;    -   Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24        carbon atoms; and    -   Group 6: fluorine, chlorine, bromine or cyano.

Illustrative examples include 9,10-di-(2-naphthyl)anthracene and2-t-butyl-9,10-di-(2-naphthyl)anthracene. Other anthracene derivativescan be useful as a host in the LEL, including derivatives of9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene.

Benzazole derivatives (Formula G) constitute another class of usefulhost materials capable of supporting electroluminescence, and areparticularly suitable for light emission of wavelengths longer than 400rn, e.g., blue, green, yellow, orange or red.

Where:

-   -   n is an integer of 3 to 8;    -   Z is O, NR or S; and    -   R and R′ are individually hydrogen; alkyl of from 1 to 24 carbon        atoms, for example, propyl, t-butyl, heptyl, and the like; aryl        or hetero-atom substituted aryl of from 5 to 20 carbon atoms for        example phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl        and other heterocyclic systems; or halo such as chloro, fluoro;        or atoms necessary to complete a fused aromatic ring; and    -   L is a linkage unit consisting of alkyl, aryl, substituted        alkyl, or substituted aryl, which conjugately or unconjugately        connects the multiple benzazoles together. An example of a        useful benzazole is 2, 2′,        2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole].

Styrylarylene derivatives as described in U.S. Pat. No. 5,121,029 and JP08333569 are also useful hosts for blue emission. For example,9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene and4,4′-Bis(2,2-diphenylethenyl)-1,1′-biphenyl (DPVBi) are useful hosts forblue emission.

Useful fluorescent emitting materials include, but are not limited to,derivatives of anthracene, tetracene, xanthene, perylene, rubrene,coumarin, rhodamine, and quinacridone, dicyanomethylenepyran compounds,thiopyran compounds, polymethine compounds, pyrilium and thiapyriliumcompounds, fluorene derivatives, periflanthene derivatives,indenoperylene derivatives, bis(azinyl)amine boron compounds,bis(azinyl)methane compounds, and carbostyryl compounds. Illustrativeexamples of useful materials include, but are not limited to, thefollowing:

X R1 R2 L9 O H H L10 O H Methyl L11 O Methyl H L12 O Methyl Methyl L13 OH t-butyl L14 O t-butyl H L15 O t-butyl t-butyl L16 S H H L17 S H MethylL18 S Methyl H L19 S Methyl Methyl L20 S H t-butyl L21 S t-butyl H L22 St-butyl t-butyl L23 O H H L24 O H Methyl L25 O Methyl H L26 O MethylMethyl L27 O H t-butyl L28 O t-butyl H L29 O t-butyl t-butyl L30 S H HL31 S H Methyl L32 S Methyl H L33 S Methyl Methyl L34 S H t-butyl L35 St-butyl H L36 S t-butyl t-butyl

R L37 phenyl L38 methyl L39 t-butyl L40 mesityl L41 phenyl L42 methylL43 t-butyl L44 mesityl

Electron-Transportinz Laver (ETL)

Preferred thin film-forming materials for use in forming theelectron-transporting layer 111 of the organic EL devices of thisinvention are metal chelated oxinoid compounds, including chelates ofoxine itself (also commonly referred to as 8-quinolinol or8-hydroxyquinoline). Such compounds help to inject and transportelectrons and exhibit both high levels of performance and are readilyfabricated in the form of thin films. Exemplary of contemplated oxinoidcompounds are those satisfying structural formula (E), previouslydescribed.

Other electron-transporting materials include various butadienederivatives as disclosed in U.S. Pat. No. 4,356,429 and variousheterocyclic optical brighteners as described in U.S. Pat. No.4,539,507. Benzazoles satisfying structural formula (G) are also usefulelectron transporting materials. Triazines are also known to be usefulas electron transporting materials.

Other Useful Organic Lavers and Device Architecture

In some instances, layers 109 through 111 can optionally be collapsedinto a single layer that serves the function of supporting both lightemission and electron transportation. Layers 111 and 111 may also becollapsed into a single layer that functions to block holes or excitons,and supports electron transportation. It also known in the art thatemitting materials may be included in the hole-transporting layer, whichmay serve as a host. Multiple materials may be added to one or morelayers in order to create a white-emitting OLED, for example, bycombining blue- and yellow-emitting materials, cyan- and red-emittingmaterials, or red-, green-, and blue-emitting materials. White-emittingdevices are described, for example, in EP 1 187 235, U.S. Ser. No.20020025419, EP 1 182 244, U.S. Pat. No. 5,683,823, U.S. Pat. No.5,503,910, U.S. Pat. No. 5,405,709, and U.S. Pat. No. 5,283,182 and canbe equipped with a suitable filter arrangement to produce a coloremission.

This invention may be used in so-called stacked device architecture, forexample, as taught in U.S. Pat. No. 5,703,436 and U.S. Pat. No.6,337,492.

Deposition of Organic Layers

The organic materials mentioned above are suitably deposited by anymeans suitable for the form of the organic materials. In the case ofsmall molecules, they are conveniently deposited through sublimation,but can be deposited by other means such as from a solvent with anoptional binder to improve film formation. If the material is a polymer,solvent deposition is usually preferred. The material to be deposited bysublimation can be vaporized from a sublimator “boat” often comprised ofa tantalum material, e.g., as described in U.S. Pat. No. 6,237,529, orcan be first coated onto a donor sheet and then sublimed in closerproximity to the substrate. Layers with a mixture of materials canutilize separate sublimator boats or the materials can be pre-mixed andcoated from a single boat or donor sheet. Patterned deposition can beachieved using shadow masks, integral shadow masks (U.S. Pat. No.5,294,870), spatially-defined thermal dye transfer from a donor sheet(U.S. Pat. No. 5,688,551, U.S. Pat. No. 5,851,709 and U.S. Pat. No.6,066,357) and inkjet method (U.S. Pat. No. 6,066,357).

Encapsulation

Most OLED devices are sensitive to moisture or oxygen, or both, so theyare commonly sealed in an inert atmosphere such as nitrogen or argon,along with a desiccant such as alumina, bauxite, calcium sulfate, clays,silica gel, zeolites, alkaline metal oxides, alkaline earth metaloxides, sulfates, or metal halides and perchlorates. Methods forencapsulation and desiccation include, but are not limited to, thosedescribed in U.S. Pat. No. 6,226,890. In addition, barrier layers suchas SiOx, Teflon, and alternating inorganic/polymeric layers are known inthe art for encapsulation.

Optical Optimization

OLED devices of this invention can employ various well-known opticaleffects in order to enhance its properties if desired. This includesoptimizing layer thicknesses to yield maximum light transmission,providing dielectric mirror structures, replacing reflective electrodeswith light-absorbing electrodes, providing anti-glare or anti-reflectioncoatings over the display, providing a polarizing medium over thedisplay, or providing colored, neutral density, or color-conversionfilters over the display. Filters, polarizers, and anti-glare oranti-reflection coatings may be specifically provided over the cover oras part of the cover.

The invention and its advantages can be better appreciated by thefollowing examples.

TRIPLET ENERGY CALCULATION EXAMPLE 1

The triplet energy of compound Inv-1 was calculated using theB3LYP/MIDI! method as implemented in the Gaussian98 (Gaussian, Inc.,Pittsburgh, Pa.) computer program. The energies of the ground state,E(gs), and the lowest triplet state, E(ts), were calculated. The energyof each state was computed at the minimum-energy geometry for thatstate. The difference in energy between the two states was furthermodified by the following equation to give the triplet energy:E(t)=0.84*(E(ts)−E(gs))+0.35. The triplet energy, E(t), of compoundInv-1 was calculated as 2.98 eV. The triplet energy of the green dopant,fac-tris(2-phenylpyridinato-N,C^(2′))iridium(III), (Ir(ppy)₃), has beencalculated by this method to be 2.55 eV and measured to be 2.53 eV. Thecalculations predict that Inv-1 will be a suitable host for the greendopant Ir(ppy)₃.

SYNTHETIC EXAMPLE 1 Preparation of Inv-2

4,4′-Bis(phenylethynyl)biphenyl was prepared by the following procedure(R×n−1). A mixture of 12.0 g (0.030 mol) of 4,4′-diiodobiphenyl, 150 mLof piperidine, and 200 mL of acetonitrile was deaerated by sparging withargon, and 0.40 g (0.6 mmol) of bis(triphenylphosphine) palladiumdichloride and 0.25 g (1.3 mmol) of copper (I) iodide were added. To theresulting stirred solution at room temperature was added 6.04 g (0.059mol) of phenylacetylene via a syringe. The reaction mixture was held atroom temperature for 30 min, and heated at 60° C. for 3 h. Theprecipitated product was collected by filtration of the cooled reactionmixture, and was washed successively with acetonitrile anddichloromethane until the solid was colorless. Yield: 8.5 g (81%). FD-MSm/z 354⁺ (M⁺).

Inv-2 (2′,2″″,3′,3″″,5′,5″″,6′,6″″-octaphenyl-p-sexiphenyl) was preparedfrom 4,4′-bis(phenylethynyl)biphenyl (R×n−2) by the following method. Amixture of 5.00 g (0.014 mol) of 4,4′-bis(phenylethynyl)biphenyl, 10.85g (0.028 mol) of tetraphenylcyclopentadienone, and 10 g of benzophenonewas heated under argon at 280° C. for 3 h. The reaction mixture wascooled to room temperature, slurried in 500 mL of dichloromethane, andfiltered. The collected product was washed successively withdichloromethane and acetone until the filtrate was colorless. Theproduct was purified by extraction with boiling benzonitrile for 30 min,followed by cooling to room temperature and filtration.

Yield: 14.0 g (93%). FD-MS m/z 1067⁺ (M+H⁺).

DEVICE EXAMPLE 1

An EL device (Sample 1) satisfying the requirements of the invention wasconstructed in the following manner:

-   -   1. A glass substrate coated with an 85 nm layer of indium-tin        oxide (ITO) as the anode was sequentially ultrasonicated in a        commercial detergent, rinsed in deionized water, degreased in        toluene vapor and exposed to oxygen plasma for about 1 min.    -   2. Over the ITO was deposited a 1 nm fluorocarbon (CFx)        hole-injecting layer (HIL) by plasma-assisted deposition of        CHF₃.    -   3. A hole-transporting layer (HTL) of        N,N′-di-1-naphthyl-N,N′-diphenyl-4,4′-diaminobiphenyl (NPB)        having a thickness of 75 nm was then evaporated from a tantalum        boat.    -   4. A 35 nm light-emitting layer (LEL) of Inv-1 and a green        phosphorescent dopant, (Ir(ppy)₃,        fac-tris(2-phenylpyridinato-N,C^(2′))iridium(III), 6 wt %) were        then deposited onto the hole-transporting layer. These materials        were also evaporated from tantalum boats.    -   5. A hole-blocking layer of        bis(2-methyl-quinolinolate)(4-phenylphenolate) (Al (Ba1q))        having a thickness of 10 nm was then evaporated from a tantalum        boat.    -   6. A 40 nm electron-transporting layer (ETL) of        tris(8-quinolinolato)aluminum (III) (A1Q₃) was then deposited        onto the light-emitting layer. This material was also evaporated        from a tantalum boat.    -   7. On top of the A1Q₃ layer was deposited a 220 nm cathode        formed of a 10:1 volume ratio of Mg and Ag.

The above sequence completed the deposition of the EL device. The devicewas then hermetically packaged in a dry glove box for protection againstambient environment.

Sample 2 was fabricated in an identical manner to Sample 1 exceptemitter Ir(ppy)₃ was used at level indicated in the Table 1. Sample 3was fabricated in an identical manner to Sample 1 except compoundIr(ppy)₃ was not included. Sample 4, 5, and 6 were fabricated in anidentical manner to Sample 1, 2, and 3 respectively, exceptbathocuproine (BCP) was used to form the hole-blocking layer instead ofAl (Ba1q).

The cells thus formed were tested for luminance, efficiency and colorCIE (Commission Internationale de L'Eclairage) coordinates at anoperating current of 20 mA/cm² and the results are reported in Table 1.TABLE 1 Evaluation Results for EL devices. Green Hole Dopant BlockingLumiance Efficiency Sample Level (%) Material (cd/m²) (W/A) CIEx CIEyType 1 6 Al(Balq) 3822 0.123 0.304 0.628 Invention 2 8 Al(Balq) 42390.135 0.307 0.628 Invention 3 0 Al(Balq) 102 0.006 0.205 0.342Comparison 4 6 BCP 3719 0.120 0.303 0.628 Invention 5 8 BCP 4303 0.1380.301 0.630 Invention 6 0 BCP 72 0.004 0.196 0.302 Comparison

As can be seen from Table 1, all tested EL devices incorporating theinvention host material demonstrated high efficiency when used with thephosphorescent dopant.

DEVICE EXAMPLE 2

An EL device, Sample 7, satisfying the requirements of the invention wasconstructed in the same manner as Sample 1 described above, except Inv-2was used in place of Inv-1. Samples 8, 9, and 10 were prepared in thesame manner as Sample 7, except emitter Ir(Ppy)₃ was used at levelindicated in Table 2. Sample 11 was constructed in the same manner asSample 7, except host Com-1 (4,4′-bis(carbazol-9-yl)biphenyl, CBP) wasused in place of Inv-2.

The cells thus formed were tested for luminance, efficiency and colorCIE coordinates at an operating current of 20 mA/cm² and the results arereported in Table 2. TABLE 2 Evaluation Results for EL devices. GreenDopant Lumiance Efficiency Sample Host Level (%) (cd/m²) (W/A) CIEx CIEyType 7 Inv-2 4 3283 0.104 0.318 0.621 Invention 8 Inv-2 6 4113 0.1310.316 0.624 Invention 9 Inv-2 8 4237 0.134 0.318 0.623 Invention 10Inv-2 0 58 0.003 0.199 0.315 Comparison 11 Com-1 6 3905 0.125 0.3130.621 Comparison

As can be seen from Table 2, all tested EL devices incorporating theinvention host material demonstrated high efficiency when used with thephosphorescent dopant. The invention host material affords a device withhigher luminance and efficiency than that the comparison host Com-1(Sample 8 vs. Sample 11).

DEVICE EXAMPLE 3

An EL device, Sample 12, satisfying the requirements of the inventionwas constructed in the same manner as Sample 8 described above, excepthost Inv-2 was replaced with a host mixture of 25 wt % of Inv-2 and 75wt % of Com-1. Sample 13 was prepared in the same manner as Sample 12,except a host mixture of 50 wt % of Inv-2 and 50 wt % of Com-1 was used.Sample 14 was constructed in the same manner as Sample 8, except hostCom-1 (4,4′-bis(carbazol-9-yl)biphenyl, CBP) was used in place of Inv-2.

The cells thus formed were tested for luminance, efficiency and colorCIE coordinates at an operating current of 20 mA/cm² and the results arereported in Table 3. TABLE 3 Evaluation Results for EL devices. HostLumiance Efficiency Sample Host1 Host2 Ratio (cd/m²) (W/A) CIEx CIEyType 12 Inv-2 Com-1 25/75  5389 0.172 0.312 0.624 Invention 13 Inv-2Com-1 50/50  5854 0.188 0.303 0.629 Invention 14 — Com-1  0/100 45790.145 0.319 0.621 Comparison

As can be seen from Table 3, the EL devices incorporating the inventivehost material in combination with the comparative host materialdemonstrated improved efficiency relative to a device using only thecomparative host material.

DEVICE COMPARATIVE EXAMPLE 4

An EL device, Sample 15, was constructed in the same manner as Sample 1described above, except host material Com-2 was used in place of Inv-1.Samples 16 and 17 were prepared in the same manner as Sample 12, exceptemitter Ir(ppy)₃ was used at the level indicated in Table 4. TABLE 4Com-2

Evaluation Results for EL devices. Green Dopant Level LuminanceEfficiency Sample (%) (cd/m²) (W/A) CIEx CIEy Type 15 6 218 0.008 0.3220.539 Comparison 16 8 224 0.008 0.322 0.542 Comparison 17 10 234 0.0080.325 0.545 Comparison

As can be seen from Table 4, all tested EL devices incorporating thecomparative host material demonstrated poor efficiency when used with agreen phosphorescent dopant.

The entire contents of the patents and other publications referred to inthis specification are incorporated herein by reference. The inventionhas been described in detail with particular reference to certainpreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

Parts List

-   101 Substrate-   103 Aanode-   105 Hole-injecting layer (HIL)-   107 Hole-transporting layer (HTL)-   109 Light-emitting layer (LEL)-   110 Hole-blocking layer (HBL)-   111 Electron-transporting layer (ETL)-   113 Cathode

1. An electroluminescent device comprising a cathode, an anode, andtherebetween a light emitting layer containing a host material and aphosphorescent light-emitting material wherein the host material isrepresented by formula (1):X′-A-X″   (1) wherein: A is selected from the group consisting of anunsubstituted phenylene ring, a biphenylene group, a terphenylene group,a naphthylene group, and a fluorene group; and each of X′ and X″ is anindependently selected aromatic group bearing an ortho aromaticsubstituent.
 2. An electroluminescent device according to claim 1,wherein: A is selected from the group consisting of an unsubstitutedphenylene ring, a biphenylene group, a terphenylene group, and afluorene group; and each of X′ and X″ is an independently selectedaromatic group bearing an ortho aromatic substituent and X′ and X″ donot contain substituents with an aromatic fused ring.
 3. Anelectroluminescent device according to claim 1, wherein thelight-emitting layer is adjacent to a layer comprising an aluminumcomplex of 2-methyl-8-hydroxyquinoline.
 4. An electroluminescent deviceaccording to claim 1, wherein A is selected from a biphenylene group, aterphenylene group, and a fluorene group.
 5. An electroluminescentdevice according to claim 1, wherein each of X′ and X″ of formula (1) isan independently selected aromatic group each bearing two ortho aromaticsubstituents.
 6. A light emitting layer according to claim 1, whereintriplet energy of the host material is higher than the triplet energy ofthe phosphorescent light-emitting material.
 7. An electroluminescentdevice according to claim 1 wherein the phosphorescent material emitsred light.
 8. An electroluminescent device according to claim 1 whereinthe phosphorescent material emits green light, and wherein the host is amaterial represented by formula (1), wherein A is selected from thegroup consisting of an unsubstituted phenylene ring, a biphenylenegroup, an o-terphenylene group, a m-terphenylene group, and a fluorenegroup.
 9. An electroluminescent device according to claim 1 containing ahost material and a phosphorescent light-emitting material wherein thehost material is represented by formula (1a),

wherein, Ar¹-Ar⁴ represent independently selected aromatic groups; andX¹-X⁶ represent hydrogen or an independently selected substituent. 10.An electroluminescent device according to claim 9, wherein the tripletenergy of the host material is higher than the triplet energy of thephosphorescent light-emitting material.
 11. An electroluminescent deviceaccording to claim 1, wherein the host material is represented byformula (1b),

wherein: Ar¹-Ar¹⁰ represent independently selected aromatic groups. 12.An electroluminescent device according to claim 11 wherein the tripletenergy of the host material is higher than the triplet energy of thephosphorescent light-emitting material.
 13. An electroluminescent deviceaccording to claim 1 wherein A is represented by a divalent form of oneof the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) independently represent substituents.
 14. Anelectroluminescent device according to claim 9, wherein thephosphorescent material emits green light and the host material isrepresented by formula (1a), wherein A is represented by the divalentform of one of the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) independently represent substituents.
 15. Anelectroluminescent device according to claim 11 wherein a phosphorescentgreen light-emitting material is present and the host material isrepresented by formula (1b), wherein A is represented by the divalentform of one of the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) represent independently represent substituents.
 16. Anelectroluminescent device according to claim 1, wherein a phosphorescentred light-emitting material is present and the host material isrepresented by formula (1), wherein A is represented by the divalentform of one of the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) independently represent substituents.
 17. Anelectroluminescent device according to claim 9 wherein thephosphorescent material emits red light and the host material isrepresented by formula (1a), wherein A is represented by the divalentform of one of the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) independently represent substituents.
 18. Anelectroluminescent device according to claim 11 wherein a phosphorescentred light-emitting material is present and the host material isrepresented by formula (1b), wherein A is represented by the divalentform of one of the following groups:

wherein, each Z^(a) is an independently selected substituent and each nis independently 0 to 4, and each m is independently 0 to 3; and R^(a)and R^(b) independently represent substituents.
 19. The device of claim1 wherein the phosphorescent material is present in an amount of up to15 wt % based on the host.
 20. The device of claim 1 wherein the hostmaterial is present in an amount of 25-75 wt % and a second hostmaterial is present in amount of 75-25 wt % of the host.
 21. The deviceof claim 20 wherein the second host material comprises a carbazole ring.22. The device of claim 1 wherein the light-emitting material is part ofa polymer.
 23. The device of claim 1 wherein the host material isrepresented by formula (1a), wherein formula (1a) is part of a polymer.24. The device of claim 1 including a means for emitting white light.25. The device of claim 24 including a filtering means.
 26. The deviceof claim 1 including a fluorescent emitting material.
 27. A displaycomprising the OLED device of claim
 1. 28. An area lighting devicecomprising the OLED device of claim
 1. 29. A process for emitting lightcomprising applying a potential across the device of claim 1.